Inverters are used for converting direct current into a grid-compliant alternating current suitable for feeding into an energy supply grid. Such inverters are used, for example, in photovoltaic systems. If galvanic isolation between the energy supply grid and the inverter is desired or even required on the basis of regulations and legal requirements for specific energy supply grids, the transformer mentioned at the outset is arranged between an AC output of the inverter and the energy supply grid. In particular in the case of relatively large photovoltaic systems which are connected directly to a medium-voltage grid, the use of transformers is imperative. In this case, it is generally additionally required that the medium-voltage grid needs to be grounded effectively. Such required effective grounding can be implemented in a technically suitable manner and inexpensively by virtue of the use of a so-called YNd transformer. In this nomenclature, the abbreviation “YN” describes a high-voltage side in star circuit configuration with a grounded neutral point and “d” describes a low-voltage side in a delta circuit configuration. In some cases, the use of such a YNd transformer is also explicitly specified by operators of the energy supply grid.
A further requirement placed on photovoltaic systems is that fault states of the energy supply grid need to be detected on the low-voltage side of the transformer, i.e. from within the photovoltaic system, and the photovoltaic system is to be disconnected from the grid after identification of specific fault states. A fault state which is relevant in this respect is in particular a single-phase phase loss, i.e. an interruption between the photovoltaic system and the energy supply grid or within the energy supply grid on one of generally three phases.
In principle, a single-phase phase loss can be detected by means of measuring devices within the energy supply grid, i.e. on the output side (high-voltage side) of a YNd transformer. The current flow can be monitored, for example, in all three phases, and a failure of a phase can be detected directly. Such a solution is very complicated and cost-intensive, however, owing to the complex measurement technology on the high-voltage side.
The document U.S. Pat. No. 4,600,961 describes an apparatus and a method for detecting high-resistance ground faults in a three-phase energy supply grid. The apparatus comprises current sensors for measuring currents flowing in the three phases, wherein positive-sequence and negative-sequence currents are determined from the measured current values by means of suitable filters, and a ratio of the negative-sequence current to the positive-sequence current is formed. A high-resistance ground fault is indicated when this ratio exceeds a threshold value assigned to it. This apparatus is also correspondingly complex owing to the necessarily complex measurement technology for measuring at the high voltages in the energy supply grid.
For feeding electrical energy into the energy supply grid, power switches in an output stage, usually an output bridge, of the inverter are actuated in a pulse-width modulation method (PWM method). For this purpose, output currents and output voltages as well as the fundamental frequency thereof are detected. Depending on a difference between the setpoint values and the actual values of the output currents, control values for the output currents are determined. Alternatively, in the case of so-called voltage-regulated inverters, a difference between the setpoint values and the actual values of the output voltages is formed and control values for the output voltages are determined. Pulse-width-modulated control signals for the power switches of the inverter are generated from the control values for the output currents or the output voltages, and the power switches are clocked correspondingly on the basis of the control signals.
Depending on the type of transformer used between the inverter and the energy supply grid, certain fault states of the energy supply grid are mapped onto the inverter-side AC voltage grid of the photovoltaic system and can be detected there and possibly taken into consideration. In the case of the mentioned YNd transformers, however, especially single-phase phase losses of the energy supply grid on the AC voltage side are not mirrored directly on the other side of the transformer, i.e. within the photovoltaic system. In the case of other transformer types, for example the so-called YNy transformers, a phase loss is mapped directly onto the side of the photovoltaic system and can be identified by known operating methods of the inverter and taken into consideration. With this transformer type, however, the precondition of effective grounding as mentioned in the outset is not provided.